An idle brain is devil's workshop

षड्दोषाः पुरुषेणेह
हातव्या भूतिमिच्छता ।
निद्रा तन्द्रा भयं क्रोधः
आलस्यं दीर्घसूत्रता ॥

- महाभारतम्

An ambitious person should overcome the following six weaknesseses - resting (oversleeping), lethargy, fear, anger, laziness and procrastination.

An idle brain is devil's workshop.
Procrastination is the thief of time.
Idleness is sweet, and its consequences are cruel.